A proposed digital transmission system consists of digitally encoded voice signals transmitted in time division multiplexed channels with plural channels devoted to conference calls between three or more (up to 128) conferees. The system also proposes the transmission of data at rates up to 128 kB/s. It permits the allocation of 1, 2, . . . data bits to a data subscriber in a chosen channel while allocating other bits of the same channel to other and different data subscribers.
Digital transmission systems that carry both PCM (pulse code modulated) voice signals and data at multiple data rates (i.e., 2.4, 4.8 and 9.6 kB/s) are, of course, known in the art; e.g., see the article "Speeding the Delivery of Data Communications" by W. P. Michaud, Jr. and S. Narayanan, Bell Laboratories Record, June 1979, pp. 163-8. The various prior art approaches share one or more of the shortcomings of the apparatus described in the cited article. For example, in the cited article, special plug-in units--called data-ports--are dedicated to a given input data terminal. A dataport is really a specialized channel unit of a channel bank, such as the D3 or D4. A dataport converts input 2400, 4800 or 9600 bit-per-second (bps) data signals into a 64,000 bps digital signal (8 bits per time slot, repeated at an 8 KHz rate). Thus, there is an inherent redundancy in the group of bits in each time slot. Further, for example, if a 9600 bps data signal is transmitted, each eight bit word of data is repeated five times. Such an approach is, of course, wasteful in terms of bandwidth. And the dedicated channel unit approach limits a transmission channel to one and only one input data signal during a given connection.
One of the stumbling blocks in achieving a more flexible approach, in the transmission of PCM encoded voice (especially in the case of conference calls) and multiplexed variable rate data signals, lies in the problem of extracting (only) the desired signal(s) from the multiplexed bit stream. This extraction operation--or demultiplexing--is particularly troublesome should one desire to use the multiple bits of a chosen channel for different data at different rates.